Concealed image development is used in self-instructional material, self-examination and multiple choice techniques for learning, testing and the like. In such self-instructional material for example, only the image corresponding to the correct answer contains one or more components for latent visual development. When properly marked with a writing substance embodying an additional component or components required for color development, the image develops thereby indicating that the correct answer has been marked. For further utilization of these concepts in concealed image development, reference can be made to U.S. Pat. No. 3,451,143.
Prior art systems, such as the ones described in U.S. Pat. Nos. 3,632,364 and 3,788,863, have utilized a three component system, comprised of a copy sheet, an ink or marking material, and an oxidation agent which is applied to the copy sheet. While these systems, and particularly the ink or marking material, have been successful when the copy sheet is produced from an acidic-based paper, the same results are not achieved when an alkali-based paper is substituted. Specifically, the marking material is ineffective in developing latent images that have been placed onto the alkali-based copy sheet. As world paper production is moving at an increasing rate toward increasing production of alkali-based paper stock, such as that employing calcium carbonate filler, it has become necessary to develop a latent image marking system that will work satisfactorily on either acidic-based or alkali-based copy sheets.
Therefore, it is an object of the present invention to provide a marking system for developing concealed images that is effective in producing a visible image from a latent image which has been placed on either acidic-based or alkali-based paper stock.
A further object is to provide a relatively non-toxic marking material for use in developing latent images on alkali-based paper stock.